The process of rotational moulding, also called rotomoulding, of plastic has been known since the 1940s for the preparation of hollow plastic articles.
This process consists of adding a thermoplastic polymer into a mould, rotating the mould so that all the points of the internal surface of the mould are in contact with the polymer while heating the mould, so as to deposit the aforementioned molten polymer on the internal surface of the mould. Thereafter, a stage of cooling allows the solidification of the plastic article, which is then removed from the mould.
Rotational moulding is advantageous because it avoids applying stress and strain to the plastic, which generally occurs in other transformations, for example in injection moulding. Indeed, the plastic does not undergo malaxation or compaction as in an extruder or in injection moulding. Rotational moulding is particularly suitable for preparing large-sized articles, such as furniture, tanks, drums, reservoirs etc.
The most commonly used polymer in rotational moulding is polyethylene, but other polymers such as vinyl polychlorides (PVC), polyamides, polycarbonates and polypropylenes can also be used. However, alone, all of these polymers tend to shrink somewhat and deform within the mould, which causes considerable withdrawal from the mould and non-uniform wall thicknesses. In addition, these polymers, used alone, are characterized either by slow coalescence, or by a raised melting point, which increases the duration of the production cycle. Polyesters, on the other hand, often lack in good thermal and mechanical properties. Thus, monolayered rotomoulded articles of polyester (e.g. PLA) are not optimal. These can be reinforced by preparing multilayered articles, for example having an additional foamed layer. However, compositions for adjacent layers have to adhere to the polyester.
There thus exists a need to produce articles by multilayered rotational moulding with polyesters, which coalesce more rapidly and which make it possible to obtain articles with minimal shrinkage and warpage and which have good adherence between the layers.
The object of this invention is thus to provide rotationally moulded (rotomoulded) articles comprising polyolefin having less shrinkage and/or warpage.
Another object of this invention is thus to provide rotationally moulded multilayered articles having good adherence between the layers.
Another object of this invention is to provide rotomoulded articles having a uniform wall thickness.
Another object of this invention is to provide rotomoulded articles presenting a good aspect of internal and external surfaces, without any bubbles, ripples, or other defects.
Another object of this invention is to provide rotomoulded articles with good mechanical properties, in particular improved impact strength.
Another object of this invention is to provide rotomoulded articles with good mechanical properties, in particular stiffness.
Another object of this invention is to provide rotomoulded articles with an easily paintable surface.
At least one of the objects mentioned above is carried out with the present invention.
The applicant found in a surprising way that by using a polyester, for example poly(lactic acid), optionally together with a polyolefin, for example polyethylene, the disadvantages mentioned above can be overcome.